The present invention relates to spindle motors for disc drive data storage devices and, more particularly, to optimized lubricating fluids for a hydrodynamic bearing within the spindle motor.
Disc drive data storage devices, known as "Winchester" type disc drives, are well-known in the industry. In a Winchester disc drive, digital data is written to and read from a thin layer of magnetizable material on the surface of rotating discs. Write and read operations are performed through a transducer which is carried in a slider body. The slider and transducer are sometimes collectively referred to as a head, and typically a single head is associated with each disc surface. The heads are selectively moved under the control of electronic circuitry to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. Each slider body includes a self-acting air bearing surface. As the disc rotates, the disc drags air beneath the air bearing surface, which develops a lifting force that causes the slider to lift and fly several microinches above the disc surface.
In the current generation of disc drive products, the most commonly used type of actuator is a rotary moving coil actuator. The discs themselves are typically mounted in a "stack" on the hub structure of a brushless DC spindle motor. The rotational speed of the spindle motor is precisely controlled by motor drive circuitry which controls both the timing and the power of commutation signals directed to the stator windings of the motor. Typical spindle motor speeds have been in the range of 3600 RPM. Current technology has increased spindle motor speeds to 7200 RPM, 10,000 RPM and above.
One of the principal sources of noise in disc drive data storage devices is the spindle motor. Disc drive manufacturers have recently begun looking at replacing conventional ball or roller bearings in spindle motors with "hydro" bearings, such as hydrodynamic or hydrostatic bearings. A hydro bearing relies on a fluid film which separates the bearing surfaces and is therefore much quieter and in general has lower vibrations than conventional ball bearings. A hydrodynamic bearing is a self-pumping bearing which generates a pressure internally to maintain the fluid film separation. A hydrostatic bearing requires an external pressurized fluid source to maintain the fluid separation. Relative motion between the bearing surfaces in a hydro bearing causes a shear element which occurs entirely within the fluid film such that no contact between the bearing surfaces occurs.
This situation is unlike typical boundary or elastohydrodynamic lubrication applications such as roller, ball and sliding bearings, where the solid bearing surfaces are often in contact with one another and sheer stress occurs at the interface. In these boundary applications, the primary purpose of the lubricant is to modify the interface to reduce wear and friction. The interfacial chemical properties of the lubricant are the most important.
In hydro bearing applications, entirely different fluid properties are of importance to the function of the bearing. The boundary properties are important only during the starting and stopping of the disc rotation. During normal operation, the most important properties are bulk properties. The hydro bearings that are being considered for use in disc drive applications are miniature units which require small power dissipation and a limited oil supply that must be adequate for a long life without escaping from the bearing. Commercially available lubricants are mostly unsuitable in some way for application in miniature hydro bearings in a disc drive.
The lubricant properties that must be controlled and the degree of control that must be obtained are unique to miniature hydro bearings for disc drives. These properties include power dissipation, viscosity and it's temperature dependence, migration, vapor pressure and evaporation rate, resistance to oxidation and corrosion, rheology, boundary properties and system compatibility. Viscosity determines power dissipation and bearing stiffness, which should be relatively constant over various operating conditions. The lubricant should have low migration so the lubricant does not creep out of the bearing. The lubricant should have a high resistance to oxidation and reactivity to provide a long life for the bearing. Rheology is the deformation and flow response to sheer.
The lubricant should also be compatible with the other materials of the disc drive. For example, migration or outgassing of the lubricant should not impair the interface between the head and the disc, such as by causing an increase in the sticking friction between the head and the disc or a degradation of the head structure or operation. Formulation of fluids for appropriate hydro bearing properties therefore requires different considerations than for fluids intended as general purpose lubricants. Another disadvantage of commercially available lubricants is that the exact composition of the lubricants is often not provided by the manufacturer and may include one or more additives that are incompatible with the sensitive operation of the disc drive.